TIJUANA  A Mexican army operation in Tijuana led to the seizure of more than $15.3 million this month and briefly lifted the veil on an illicit and highly secretive activity: the shipment of large amounts of U.S. currency that keeps drug-trafficking groups in business.

Mexican and U.S. authorities have stepped up efforts in recent years to follow the trail of these dollars, which are the proceeds of drug sales in the United States.

Known as bulk cash, the money allows criminal organizations to buy weapons, bribe government officials, dig cross-border drug tunnels, pay hit men — whatever it takes to continue the northbound flow of drugs.

But in a drug trade that’s measured in the billions of dollars, experts said those efforts have done little to cripple the traffickers’ ability to operate.

Seizures such as the new one in Tijuana “are valuable in signaling to the public that law enforcement is active and working hard. But they are a notoriously bad indicator of success since seizures go up and down dramatically, and there is no clear indication that they have any long-term impact on the overall market,” said Eric Olson, a senior associate at the Mexico Institute of the Woodrow Wilson Center in Washington, D.C.

Olson said reasonable estimates peg the value of Mexico’s export drug market at $7 billion to $9 billion per year, though there are far higher figures cited. The U.S. domestic drug trade generates tens of billions of dollars annually, according to the National Drug Intelligence Center.

The cash crosses in both directions — smuggled south into Mexico in compartments of vehicles and increasingly, U.S. authorities suspect, brought north to the United States in shipments that are declared at ports of entry as legitimately earned.

During the past two years, Mexican authorities have tried to crack down on southbound smuggling of cash, weapons and other contraband by using an electronic inspections system at the ports of entry. They have not said what level of success, if any, that strategy has yielded.

In 2010, Mexico’s federal government also launched a campaign to combat money laundering by restricting deposits of U.S. currency into Mexican banks. Since then, U.S. authorities have noticed a sharp spike in the northbound flow of dollars. They suspect that because drug traffickers no longer can store large amounts of money in Mexican banks, they are sending it back to the United States.

In Mexico, “it’s just stockpiling, it’s a problem for them. What do they do with it? How do they get it back into pesos to further their operations?” said Michael Carney, deputy special agent in charge of Immigration and Customs Enforcement investigations in San Diego.

ICE figures show a $300 million rise in declared currency heading from Mexico into California from 2009 through last year. A similar increase was registered along New Mexico’s stretch of the U.S.-Mexico border, while Arizona’s was far smaller ($1.9 million).

“The difficult part for us is proving the source of that cash,” Carney said. “You’ve got all this cash coming up, commingled with legitimate profits from legitimate businesses.”